So sorry to hear about your ongoing plant problems. The description doesn't point clearly to any one cause. My best guess is a root rot organism is affecting the plants. These are often brought on by soggy soil conditions but not always. Then there is the possibility of some root feeding insects such as grubs or wire worms.
You need someone to diagnose a plant sample or photos. I suggest you take a sample to your County Extension Office and see if they can identify the problem and if necessary prescribe an appropriate remedy. They may need to send it to the state lab so call them first to get instruction for what type of sample to bring in.
Thanks for the question. Please stop in again soon!
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